1. Field of the Invention
The present invention relates to a light-scattering substrate, a method for manufacturing a light-scattering substrate, a polarizing plate and an image display device.
2. Description of the Related Art
In recent years, a liquid crystal display device (LCD) has been widely used because it is slim, lightweight and low in a consumed power. The liquid crystal display device includes a liquid crystal cell and a polarizing plate. The polarizing plate is in general composed of a protective film and a polarizing film, and it is obtained by dyeing a polarizing film made of a polyvinyl alcohol film with iodine, stretching the film and then laminating the both surface thereof by protective films. In a transmission liquid crystal display device, this polarizing plate is installed on the both sides of a liquid crystal cell, and one or more optical compensatory sheets may be further disposed. Also, in a reflection liquid crystal display device, in general, a reflecting plate, a liquid crystal cell, one or more optical compensatory sheets and a polarizing plate are disposed in this order.
Since LCD is not a spontaneous emission-type display device, it requires a surface light source. As an embodiment of the surface light source, a backlight type in which a member having light diffusion capability, such as a diffusion sheet and a prism sheet, or the like is interposed between a liquid crystal cell and an emission light source, thereby forming a uniform surface light source is widely used. As the emission light source, a cold cathode fluorescent lamp (CCFL) or LED is used. Also, in a part of LCDs, there is known an embodiment (edge light type) in which a light source is disposed in an edge portion of a light guide plate, and a surface light source is realized through a combination with a diffusion sheet, a prism sheet or the like. As described previously, in these devices, since the surface light source is in general converted from a linear light source or a point light source, a uniform surface light source is realized using a diffusion sheet or a diffusion film (light diffusion sheet or light diffusion film).
Among the members of a backlight for obtaining a surface light source, the diffusion sheet is in general disposed between the backlight and the polarizing plate on the backlight side. By disposing the diffusion sheet, not only a reduction of brightness unevenness due to a light source and surface uniformity of a display characteristic can be achieved (see JP-A-2000-75134), but the generation of interference-induced stripes such as moire to be caused by interference of incident light with pixels in the liquid crystal cell can be suppressed.
However, in recent years, it has been attempted to reduce the number of members of a liquid crystal display device or to decrease the number of fluorescent lamps which are used for the light source for the purpose of achieving a low consumed power. Also, for the purpose of thinning LCD, a distance between the backlight light source and the diffusion sheet becomes short so that in related-art diffusion films, it becomes difficult to achieve uniform light diffusion. Then, in order to gain the distance as far as possible, a material having diffusibility on the surface of the polarizing plate on the backlight side is being used as a substitution of the diffusion sheet.
For example, JP-A-2000-75134 proposes a light diffusion polarizing plate having a light diffusion layer having prescribed characteristics, in which an porous amorphous particle and a spherical particle are dispersed and contained and discloses that the light diffusion sheet can be omitted thereby. Also, JP-A-2001-172403 and JP-A-2003-25504 propose a method for manufacturing a light diffusion film including casting a dope containing a fine particle on a support and discloses that according to this method, a light diffusion film which is excellent in optical isotropy or the like can be prepared.